The present invention relates to a method for treating a gas by adsorption, of the type in which a PSA (pressure swing adsorption) treatment unit is used and in which said treatment unit is made to follow a nominal operating cycle defined according to the nominal operating conditions and for the purpose of guaranteeing minimum treatment performance levels.
PSA units are widely used for the separation and/or purification of feed gases, especially in fields such as the production of hydrogen and carbon dioxide, drying, separation of the constituents of air, etc.
“H2-PSA” units, which produce substantially pure hydrogen, are used with feed gases of varied origin, these being formed for example from gases resulting from steam reforming, from a refinery or from a coke oven, or else formed from waste gases coming from ethylene or styrene production units, or from cryogenic hydrogen-carbon monoxide separation units.
In general, a PSA unit consists of several adsorbers that follow, shifted in time, an operating cycle, hereafter called for convenience a “PSA cycle”, which is distributed uniformly over as many phase times as there are adsorbers in operation and which is formed from basic steps, namely the following steps:                adsorption at substantially a high pressure of the cycle;        cocurrent depressurization, generally from the high pressure of the cycle;        countercurrent depressurization, generally down to the low pressure of the cycle;        elution at substantially a low pressure of the cycle; and        repressurization from the low pressure of the cycle up to the high pressure of the cycle.        
Hereafter, the description relates to the operation of a PSA unit in the steady state, that is to say away from the transient periods during which the unit is started up or shut down, which generally correspond to special cycles set up for this purpose.
The main operating constraint of a PSA unit in the steady state consists of the purity level of the product. Under this operating condition, the treatment performance levels of a PSA unit are then generally optimized, either to maximize the extraction efficiency (amount of gas produced/amount of this gas present in the feed gas), or to minimize the energy consumed.
What is obtained is as it were a nominal operating cycle of the PSA unit, defined directly according to the nominal operating conditions (flow rate of the feed gas, flow rate of the treated gas, composition of the feed gas, operating temperature of the unit, etc.).
When the operating conditions depart only slightly from the nominal conditions, it has been proposed in the past to regulate the operation of the PSA unit by adjusting one or more parameters of the nominal cycle so as to guarantee that the treatment performance levels remain above predefined minimum limits. Two regulations employing this approach are:                “capacity” regulation, which consists in modifying the duration of the phase time of the cycle according to the variation in the feed gas flow rate; and        “purity control” regulation, which consists in modifying this phase time according to the purity of the gas treated.        
However, when the operating conditions depart greatly from the nominal operating conditions, the operation of the PSA unit is degraded, the expected performance levels no longer being achieved.